The first International Workshop on Fetal Genetic Pathology at Kletkamp (organized by Profs H. Rehder, then of Lubeck, and Enid F. Gilbert, Madison) followed the VIII ICHG in Berlin and was so successful that a repeat has been demanded by many individuals to follow the VIII ICHG, Washington DC, in Oct. 1991. The purpose of the proposed Big Sky Workshop is two-fold: A.) To focus on the earliest stages of (human) development and its genetic and teratogenic abnormalities (the "'primary field"); and B.) To bring together experts in basic sciences investigating these early stages of development in humans and other vertebrates, birth defects epidemiologists with particular expertise in associations, teratologists, and clinicians who are involved in the daily care and study of humans with defects of earliest development from a pathological, embryological and clinical-genetical perspective. Subjects of greatest pertinence to these early stages of development are: Homeoboxes and their role in midline development and segmentation (to be discussed by Frank Ruddle, Yale); 2.) Retinoic acid and retinoic acid receptors in midline and limb development (Gregor Eichele, Harvard); 3.) Segmentation in the CNS (Andrew Lumsden, London, UK); 4.) Associations (Jane Evans, Winnipeg); 5.) Effects of aneuploidy and imprinting on earliest embryonic and placental development in humans (Dagmar Kalousek, Vancouver); 6.) Conjoined twinning (Geoffrey Machin, Edmonton); 7.) Teratogenic disruptions of gastrulation in mammals and a model of caudal regression (Kathy Sulik, Chapel Hill); 8.) Abnormal development of umbilical cord and abdominal wall (Enid Gilbert-Barness, Madison); 9.) overview with special attention to mutations affecting midline development (Opitz, Helena). In addition to these invited talks there will be 2 1/2 sessions on proffered papers on the main subject, and 3 days of poster exhibits on any aspect of fetal genetic pathology. It is proposed to publish the Proceedings with additional chapters on related subjects for a wide audience of pediatric pathologists, fetal morphologists, embryologists, teratologists, developmental geneticists, and clinical geneticists.